A fuel cell stack is a power generation system which oxidizes a fuel through an electrochemical process to thereby directly convert energy released due to such oxidization reaction into electric energy. Such fuel cell stack has a membrane-electrode assembly in which a polymer electrolyte membrane, which selectively transports hydrogen ions, is sandwiched by a pair of electrodes made of porous materials. Each of the pair of electrodes includes: a catalyst layer that contains, as a main ingredient, carbon powder supporting a platinum-based metal catalyst and contacts with the polymer electrolyte membrane; and a gas diffusion layer formed on a surface of the catalyst layer, the gas diffusion layer having both air permeability and electronic conductivity.
In fuel cell systems of this type, when a cell continues to be operated within an operation zone where the cell voltage reaches an oxidation voltage (about 0.7 V to 1.0 V), an oxide film may be formed on a surface of the platinum catalyst in the catalyst layer and reduce an effective area of the platinum catalyst, which may cause degradation of output characteristics. In view of these circumstances, Patent Document 1 includes descriptions regarding processing in which, if the electric power requested to be generated by the fuel cell is less than a predetermined value, the supply of air (oxidant gas) to the fuel cell stack is stopped and the output voltage of the fuel cell stack is forcibly decreased by a DC/DC converter so that the cell voltage is lowered to a reduction voltage (e.g., 0.6 V or lower) to thereby remove an oxide film from the surface of the platinum catalyst and recover the performance of the catalyst layer (such processing will hereinafter be referred to as “refresh processing”).
Patent Document 1 also describes, with regard to a fuel cell vehicle which uses the fuel cell system as an in-vehicle power supply, prohibiting the refresh processing if the fuel cell vehicle is traveling at a speed equal to or greater than a predetermined value.